In the absence of a loading dock, loading and unloading large transport vehicles presents a substantial problem for a standard forklift truck because of the limited reach of such a truck. Even if the reach is sufficient to unload the vehicle, there can be a problem in transporting the forklift truck along with the load because standard forklift trucks are bulky and heavy and do not tow well.
Lutz et al., U.S. Pat. No. 4,460,064, issued Jul. 17, 1984, discloses a forklift truck having sets of wheels at the front, center, and rear. During normal use, the truck is supported on the center and front sets of wheels, but when it is desired to load the truck to a higher surface, the forks are lowered onto such surface, the front wheels are retracted and supports for the rear wheels are telescoped downward to raise the body of the truck as the forks are lowered relative to the body.
Steiger et al., U.S. Pat. No. 4,180,363, discloses a specialized track and carriage for the bed of a transport vehicle and a cooperating attachment for a forklift to assist loading a forklift truck onto and off of the bed.
Netherlands Patent No. 8005192, dated Sep. 17, 1980, discloses a forklift truck having a chassis collapsible to compact condition for transport. The chassis is extendible to a more stable work position. However, this patent is not concerned with loading and unloading of a forklift truck between different levels.